OP THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 343 



Paramelania. In this respect the regulation Purpurina bellona (which 

 occurs on a higher horizon than P. aspera) more resembles the average 

 Paramelanias of Tanganyika. It should be remarked also that most 

 species of Paramelania have a considerable amount of brown scaly or 

 epidermal matter, and are generally thick and nassoid or purpuroid in 

 the texture of the shell. Eeference is made .to the conchological 

 similarity of Pyrgulifera^ a genus of fresh -water shells (p. 343) of the 

 Upper Chalk, to Paramelania^ and this casual identification opens up 

 several interesting questions. 



7. Na^sopsis nasm, Smith* with Purpurina inflata {? auctor). 



See pp. 250 and 347. During life this mollusc, we are told, inhabits 

 the surface rocks of Tanganyika, and its shells are always richly 

 encrusted with the green algse which clothe the rocks for a considerable 

 depth. It is sluggish and appears to browse within a very limited area, 

 like the Patellas of the Ocean beach. 



As regards the Jurassic fossil figured for comparison (upper figures, 

 p. 347) under the name of Purpurina inflata^ I should point out that this 

 specimen is not Purpurina inflata^ Tawney, but a peculiar unnamed form 

 which was figured in Plate I of the " Jurassic Gasteropoda." The true 

 P. infiata has a very diff"erent figure and ornamentation, but possesses a 

 rounded and almost unchannelled aperture, having in fact the least 

 indented mouth of all the Piirpurinoe. 



The real value of these comparisons is an unknown quantity, bat the 

 conchological resemblance of both Paramelania and Nassopsis to certain 

 named and unnamed forms of Piirpm-ina is clearly pointed out by 

 Mr. Moore, and admitted, as I understand, by Mr. Edgar Smith. Such 

 resemblances are interesting, but if Paramelania and Nassopsis are really 

 diflFerent genera, as their internal structure would imply, one learns two 

 things from this fact : (1) that the outward form of the shell is not 

 always indicative of the character of the animal within, and (2) that two 

 different genera of existing molluscs are compared with the one Jurassic 

 genus, Purpurina. 



It may be mentioned here that the genus Purpurina was somewhat 

 loosely constituted by D'Orbigny, and was more carefully reconstituted 

 by Piette and Deslongchamps, who regarded it as having relations on the 

 one side with Turbo and on the other with Cerithium and Purpura, 

 Fischer places Purpurina among the Littorinidse, but its real family 

 relationship is by no means clear. In the Jurassics of this country 

 Purpurina first makes its appearance in the Marlstone (Middle Lias), 

 culminates in the Inferior Oolite, is rare in the Great Oolite, and dies out 



* I possess a specimen supplied by Sowerby and Fulton, marked 

 Pa/ramelania coronata^'' Bourguignat ; which greatly resembles the 

 figures of Nassopsis nassa. 



